The present invention relates to telecommunications systems and, particularly, to an improved method for converting signals between A-law and xcexc-law format.
It is conventional in telecommunication systems to digitize voice signals according to a predetermined encoding law. Currently, there are two international standards for encoding pulse code modulated (PCM) signals. In the United States and Japan, xcexc-law encoding is used; in Europe and the rest of the world, A-law encoding is used. Both standards are promulgated by the International Telecommunications Union (ITU) Telecommunication Standardization Sector in the ITU-T Recommendation G.711, which is hereby incorporated by reference in its entirety as if fully set forth herein.
When a call is placed between countries using different PCM encoding schemes, the network must provide conversion through the use of a device that converts xcexc-law to A-law and A-law to xcexc-law. Typically, the device maintains a map or look-up table to perform the conversion. Such maps are defined at Tables 3 and 4 of the Recommendation G.711. The nature of the encoding, however, is such that distortion may nevertheless result when the signals are decoded.
For example, FIG. 1A illustrates an exemplary digitized voice wave form 1a that is to be sent from a starting location that uses xcexc-law PCM encoding. When xcexc-law converted, the digitized voice wave form 1a may take on values (represented by the waveform 1b of FIG. 1B) that are slightly higher than in the original wave form. At the destination location that uses A-law PCM encoding scheme, the xcexc-law converted wave form is then converted to A-law format. The A-law converted wave form may take on values (as shown in the waveform 1c of FIG. 1C) that are even higher than in either the xcexc-law converted wave form or the original wave form. The A-law converted wave form 1c is then A-law decoded, and these cumulative changes in the wave form values due to the xcexc-law and A-law conversion can ultimately lead to distortion. Similarly to the above example except in reverse, there can be distortion when a signal wave form is received when sent from a starting location that uses A-law coding to a destination location that uses xcexc-law coding. This type of signal distortion is only an annoyance when dealing with voice signals, but the distortion when dealing with data signals can impair the receiving modem""s ability to demodulate accurately.
Thus, there is a need for improved xcexc-law to A-law conversion and A-law to xcexc-law conversion, both for data and voice communications.
These and other disadvantages in the prior art are overcome in large part by a system and method according to the present invention. A modem or other telephony device is configured to identify (or receive notification of) the companding law used by a destination modem or telephony device. The telephony device can then, using stored conversion tables, predict the inverse mapping from the converted encoding law. Having done so, the telephony device can identify distorted portions of the decoded signal and modify the original signal before it is sent to the encoding or encoding-converter unit.
A telephony-over-LAN (ToL) client according to an embodiment of the invention includes a digital signal processor (DSP) and a memory for storing A-law and xcexc-law maps. The ToL client receives an identification from its gatekeeper of the companding law used by the destination. The ToL client then optimizes the signal for that law. Thus, the ToL client may generate a different, optimized amplitude digital signal if the destination uses a different companding law than the ToL client""s location does.
A modem according to an embodiment of the invention similarly is configured to identify the destination device""s encoding law. The modem may then adjust its output analog signal (either directly or by adjusting the input digital signal) to xe2x80x9ctrickxe2x80x9d the central office or PBX into choosing a different signal level (different than the signal level resulting normally without use of the invention) when it converts.
These and other embodiments will be better understood from the following detailed description in conjunction with the following drawings.